Ultrafine aerosol size distributions (~2.7 to 10 nm diameter)
and gas phase sulfuric acid concentrations were measured from
the NCAR C-130 aircraft as part of the Southern Hemisphere Marine
Aerosol Characterization experiment (ACE-1). This data can be
used to estimate ultrafine particle formation rates and ultrafine
particle growth rates. These observed rates, in turn, can be
compared with the formation and growth rates of sulfuric acid
droplets calculated from measured sulfuric acid concentrations.
Our earlier work at remote marine and continental sites have
shown that observed ultrafine particle formation and growth rates
were much higher than those predicted for sulfuric acid droplets.
We have speculated that the participation of ammonia may account
for the enhanced particle formation rates and that condensation
of additional species, like organics, could explain the higher
growth rates.

During ACE-1, evidence of recent new particle formation was frequently
observed in the out-flow regions of clouds. However, the highest
concentrations of particles smaller than approximately 4 nm diameter
and most dramatic evidence of recent new particle formation were
recorded downwind of the southern tip of Macquarie Island, the
site of a large penguin colony and a significant source of ammonia.
Despite unusually high nucleation rates, sulfuric acid concentrations
were typical of levels recorded in cloud out-flow regions, approximately
8x106 molecules cm-3. Closest to the southern tip of the island,
the ultrafine particle distribution appeared to peak below the
ultrafine condensation particle counter lower detection limit
of ~2.7 nm; further downwind from this point the ultrafine mode
was observed at larger particle diameters suggesting that the
particles were growing as they moved away from the island. In
this paper, ultrafine particle formation and growth rates downwind
of Macquarie Island will be discussed.